Electrostatic rotary paint atomizers are often used for painting workpieces in industrial applications. The paint is atomized by flowing over the inside of an enlarging diameter conical or bell shaped surface as the surface is rotated at a very high speed. When the paint reaches an outer annular edge of the surface, it is thrown from the edge and atomized through the action of centrifugal force. At the same time the paint is atomized, it is subjected to a very high voltage relative to the workpiece which imparts an electrostatic charge on the atomized paint. The electrostatic charge attracts the atomized paint to the workpiece to provide a high coating transfer efficiency. Sometimes the atomized paint also is subjected to a flow of air surrounding the atomizing device to further aid in directing the atomized paint to the workpiece. Electrostatic rotary atomizers are known for their high transfer efficiency, high paint throughput and for the application of a high quality coating.
The design of the rotary atomizing device is both a science and an art. Very small changes in the design can have a profound effect on the quality of the applied coating. For coating large surface areas such as automobile bodies and where larger paint throughputs are required, the device may have a diameter of about 3 inches (about 7.5 cm) at the paint discharge edge. For smaller surfaces and where lower paint throughputs are required, a smaller device having an annular paint discharge edge with a diameter of about 1 inch (about 2.5 cm) may be used. The device is mounted on a shaft of a motor, such as an air driven turbine, for being rotated at high speeds which may range, for example, from 10,000 rpm to 40,000 rpm or more. Paint is delivered from a source through a feed tube to an internal chamber in the device. For some devices, the paint is delivered by a feed tube located to one side of the turbine shaft. For other devices, the turbine shaft is hollow and the paint is delivered through a stationary feed tube extending coaxially through the turbine shaft. Paint fed to the internal chamber attaches itself either to an outer annular surface of the chamber or to a front wall surface of the chamber. The paint is accelerated to the speed of the adjacent surface and tends to move outwardly through centrifugal force. Preferably, the surfaces are sloped to direct the paint flow towards a series of uniformly spaced openings through the front surface of the chamber. The paint flows through the openings and attaches itself to the conical or bell shaped surface which leads to the annular paint discharge edge at the front of the device.
When paint is fed to the internal chamber in the atomizing device along the axis of the device, the chamber is preferably closed at its rear by a radially inwardly directed rib which is spaced close to the stationary paint feed tube. The rib is necessary to prevent paint from splashing back or flowing into the annular opening between the rotating drive shaft and the stationary paint feed tube. If paint should collect in this annular space, it generally would not be removed by automatic cleaning cycles in which paint supplied to the device is replaced by solvent. The paint would eventually harden and could interfere with operation of the atomizer. Manual cleaning of the device is time consuming since it requires removal of the atomizing device from the turbine shaft. It is sometimes necessary to remove the device from the turbine shaft for cleaning the internal chamber and the small passages connecting the chamber with the front of the device to remove obstructions. However, the rib which forms the rear surface of the paint receiving chamber makes it difficult to manually clean the chamber and the passages. Cleaning the internal chamber is especially difficult for smaller diameter atomizing devices where the opening into the chamber is quite small. The rib also makes the atomizing device expensive to manufacture. In the past, it has been necessary to manufacture the device by machining from a solid block of a material such as aluminum. The internal rib which formed the rear wall of the paint receiving chamber prevented effectively molding or casting the device, which could significantly reduce the manufacturing cost of the device.